dd元素的z-index最高，但是不能覆盖#CC元素。

HTML: <div id="aa">   <div id="bb">     <div id="dd"></div></div>   <div id="cc"></div> </div>  CSS: #aa{   position: absolute;   width: 100px;   height: 100px;   background-color: #ddd;   z-index: 1; } #bb{   position: absolute;   width: 80px;   height: 100px;   background-color: #000;   z-index: 2; } #cc{   position: absolute;   width: 70px;   height: 10px;   bottom:0;   background-color: red;   z-index: 2; } #dd{   position: absolute;   width: 60px;   height: 2000px;   background-color: yellow;   z-index: 17; }